Adsorption methods for removing trace contaminants from a flowstream typically comprise passing the flowstream over or through a sorbent structure. The sorbent structure may be defined by a plurality of pellets or an array of tubes or plates or the like, and such structure typically is positioned within the flowpath of the flowstream to be treated. The sorbent structure may comprise, or be coated with, sorbent particles that adsorb targeted impurities from the flowstream. Although such systems are well known in the art, several problems or shortcomings associated with conventional adsorption methods correspondingly also are well known in the art.
For example, when the sorbent becomes saturated, the sorbent must be regenerated or removed and replaced. Typically, the entire sorbent structure simply is replaced. Preferably, the sorbent structure is regenerable. In some systems, the sorbent structure is removed from the adsorption stream, subjected to a desorption process, and then re-exposed to the adsorption stream. One alternative method is described in U.S. Pat. No. 6,712,878 to Chang, et al., wherein sorbent particles are injected into the flowstream and then the flowstream is passed into contact with the sorbent structures. The saturated sorbent periodically is removed and fresh sorbent again is injected into the flowstream.
Another problem associated with conventional adsorption methods is the efficiency of the adsorption technique employed. Often, the unique characteristics of the targeted impurities and the sorbent itself dictate that the adsorption process operate within a desired temperature range. Several methods are known for raising the temperature of the process including heating the flowstream or the sorbent structure by employing an auxiliary heat source. However, non-uniform heat distribution within a fixed-bed substrate or other sorbent structure negatively impacts the efficiency of the process. In addition, the time it takes for an auxiliary heat source to raise the temperature of the sorbent structure, and thereby raise the temperature of the sorbent and the working fluid, further negatively impacts the efficiency of the process. Moreover, less-complex auxiliary heat sources may not provide the capability to reach and hold a narrow operating temperature range as may be required for the subject adsorption goal. Although more complex auxiliary heating systems may be capable of achieving and holding a narrow operating temperature range in a comparatively short time interval, such devices add considerable weight and cost to the adsorption process.
In addition to conventional applications for adsorption processes, such processes occupy an important niche in spacecraft environmental control and life support systems. Primary applications for adsorption processes exist in the area of cabin air quality control. Since the beginning of crewed space exploration, adsorption processes have been at the forefront for ensuring that cabin air is suitable for the crew to breathe by removing trace chemical contaminants and CO2. The ability to remove trace contaminants (e.g. alcohols, ketones, aromatics, halocarbons, and ammonia) from cabin air is a necessary aspect of spacecraft life support systems such as that employed on the International Space Station (“ISS”). Currently, this trace contaminant control system (“TCCS”) requirement is met on the ISS by employing a 50 lb. bed of acid-treated activated carbon, which is not regenerated. Due to its long life (>2 yrs.), the carbon bed is simply replaced periodically. The current CO2 removal system on the ISS employs two pellet bed canisters of 5A molecular sieve that alternate between regeneration and sorption via heating and exposure to space vacuum.
It is anticipated that adsorption processes will continue to remain at the forefront of spacecraft cabin air quality control technologies. As mission durations increase and exploration goals reach beyond Earth orbit, the need for regenerable adsorption processes becomes paramount. Thus, there is a need in the art for an adsorption process that is capable of regenerably removing trace contaminants from a flowstream in an efficient, cost-effective, and robust manner suitable for conventional applications as well as for aerospace applications. There also is a need in the art for an adsorption process that is capable of regenerably removing CO2 from a flowstream in an efficient, cost-effective, and robust manner suitable for conventional applications as well as for aerospace applications.